Waves are a product of the interaction between wind and water. The critical factor in determining wave height is not wind speed, but the distance over which the wind blows. It takes time to build up large waves and the longer the reach of the wind, the higher the waves. Starting from a flat calm sea, there is an orderly progression of events that produce waves that lead from still water to a high rolling swell. Wind blowing across a flat smooth sea hardly catches the water at all. In time, however, it begins to set up a faint pattern of ripples that grip the wind, gradually giving rise to short choppy waves that readily catch the wind. For the first ten hours, those small waves grow rapidly, but after thirty hours, they hardly grow at all.
In the open ocean, waves come in all shapes and sizes, and in different frequencies. Some are little more than ripples that have only just begun to form. Others are left from storms that may have occurred several days before. As the wind continues to drive the waves, however, these confused patterns of waves and ripples slowly begin to sort themselves. Some cancel out as the crest of one wave collides with the trough of another. Others, however, begin to slowly combine as crest meets crest, building up a chain of large waves with nearly identical shapes that move through the ocean with a regular rhythm, known as sea swell.
Once that pattern of a swell is set up, waves can move and gradually grow in height. As those incoming waves slow down, they also begin to change direction. As the waves move closer to shore they continue to grow in height and slow down as the water shallows until they reach some critical point of instability and break. High waves can break and reform several times before they reach shore. Along the beach, each breaking wave sends a turbulent surge of water flowing across the sand. Like a fast-moving stream, that moving water carries grains of sand up and down the face of the beach.
These water movements take place not only on land, but underwater as well. Walking knee-deep through the surf you can feel the pull of the waves against your legs and feel the sand shift under your feet. The beach includes not only the dry sand above water, but the sand underwater as well up to a depth of several meters. Waves and currents are continually carrying sand back and forth between these wet and dry worlds of the beach. While an individual grain of sand can move back and forth several times a day, the net direction of movement changes from season to season as the beach alternately grows and shrinks. In the summer, the net flow of sand is inland. You can see it in the broad, flat surface of the summer beach. The reason for this build up has to do with the average shape and the strength of waves during the summer. It is well known that this moving water or shifting waves have substantial energy which is difficult to harness.